Electric motors can convert electrical energy into mechanical energy through electromagnetic interactions. The electric motors are widely used in daily life and applied to many fields. For example, the electric motors can be applied to machine tools, water pumps, light machineries, wind power systems, hydroelectric power generation systems and the like. The electric motors are fundamental machines in industry and closely linked with economic development.
Regarding to energy saving issues, International Electrotechnical Commission (IEC) provides the energy efficiency standards for electric motors, for example, IE1 to IE4, even to IE5, from low to high. In order to achieve the high efficiency standards, induction electric motors mostly use high magnetically conductive materials or high level magnetically conductive steels, but the increase of efficiency has hit the bottleneck. As a result, some manufactures stop develop the induction electric motors, they start to put rare earth materials and actuator into an electric motor in order to achieve level IE5, but the overall cost is largely increased. In addition, the rare earth materials have potential problems of demagnetization, and such electric motors cannot be directly connected to electric supply. Thus, the developers are constantly trying to develop a high efficiency and cost effective electric motor.